1. Field of the Invention
The present invention relates to an abnormality diagnosis apparatus for an internal combustion engine, which apparatus determines whether there is an abnormality in a fuel system based on an output from an exhaust gas sensor mounted on an exhaust duct of the internal combustion engine.
2. Description of Related Art
For example, JP-A-2001-329894 (for example, P. 2) describes an air-fuel ratio control system that performs a abnormality diagnosis of a fuel system (for example, fuel injection valve, fuel pump) for an internal combustion engine. The air-fuel ratio control system establishes an air-fuel ratio feed-back correction amount for a feed-back control based on an output from an exhaust gas sensor (air-fuel ratio sensor, oxygen sensor), which is mounted on an exhaust pipe of an internal combustion engine, in order to cause an actual air-fuel ratio estimated based on exhaust gas to generally match with a target air-fuel ratio. Also, the air-fuel ratio control system learns a different amount from the target air-fuel ratio to the actual air-fuel ratio as a learning correction amount, and the air-fuel ratio control system controls a fuel injection quantity by using the air-fuel ratio feed-back correction amount and the learning correction amount. Thus, the air-fuel ratio control system determines whether there is the abnormality in the fuel system based on a total value of an air-fuel ratio feed-back correction amount and a learning correction amount.
The internal combustion engine includes a gasoline engine that uses gasoline or a diesel engine that uses light oil. Recently, there have been constructed more self-service gas stations, where a driver refuels a vehicle by himself or herself. Therefore, the driver may supply light oil to a vehicle having the gasoline engine by mistake. In the above case, the light oil is other fuel that is not usable for the gasoline engine. Also, bad fuel, which is intentionally made by mixing other fuel into gasoline, may be supplied to the vehicle. In the above, the other fuel for the gasoline engine may be light oil or kerosene. When the other fuel contaminates supply fuel, which is supplied to the engine, an air-fuel ratio of the mixture supplied to the engine or oxygen concentration in the exhaust gas changes, and thereby the output from the exhaust gas sensor may changes. Accordingly, the air-fuel ratio feed-back correction amount is changed in accordance with the above change.
However, as shown in JP-A-2001-329894 (for example, page 2), in the abnormality diagnosis of the fuel system by using the air-fuel ratio feed-back correction amount, when the contamination of the other fuel causes the air-fuel ratio feed-back correction amount to widely change, an abnormality determination parameter, which is based on the air-fuel ratio feed-back correction amount, may exceed an abnormality determination threshold value even when the fuel system operates in a normal state. Accordingly, there is a possibility of misdiagnosis for erroneously determining that there is the abnormality in the fuel system disadvantageously. In the above, the abnormality determination parameter may be a total value of the air-fuel ratio feed-back correction amount and the learning correction amount, for example.